Article 3.4 of the Kyoto Protocol refers specifically to agricultural soils.
However, the basic principles discussed herein cover soil carbon dynamics in
general regardless of the land use and apply to all ecosystems (cropland, forest,
grazing lands, and wetlands).

Although most carbon enters ecosystems via leaves, and carbon accumulation
is most obvious when it occurs in aboveground biomass, more than half of the
assimilated carbon is eventually transported below ground via root growth and
turnover, exudation of organic substances from roots, and incorporation of fallen
dead leaves and wood (litter) into soil. Soils contain the major proportion
of the total ecosystem carbon stock in all ecosystems.

Carbon inputs to soil are determined by the amount and distribution of primary
production, the life cycle of the vegetation, and exogenous organic matter additions
(e.g., composts, manure). Thus, practices that increase net primary production
(NPP) and/or return a greater portion of plant materials to the soil have the
potential to increase soil carbon stocks. Organic matter decomposition is influenced
by numerous physical, chemical, and biological factors that control the activity
of microorganisms and soil fauna (Swift et al., 1979). These factors
include the abiotic environment (temperature, water, aeration, pH, mineral nutrients),
plant residue quality (i.e., C:N ratio and lignin content), soil texture and
mineralogy, and soil disturbance (tillage, traffic, logging, grazing, etc.).
The root system, depth distribution, and chemical characteristics of the root
biomass also play significant roles in SOC dynamics (Gale and Cambardella, 1999).
Practices that reduce the decomposition rate by altering these physical, chemical,
or biological controls also lead to carbon storage.

In some soils, changes in soil inorganic carbon (SIC) stocks can be significantly
influenced by land use and management (Lal et al., 2000), making SIC
either a sink or source of atmospheric CO2:

In native ecosystems, the weathering of base-rich silicate minerals releases
calcium or magnesium, which can combine with CO2
and precipitate as secondary carbonates (CaCO3
or MgCO3). The addition of excess calcium or
magnesium (e.g., in sewage sludge) to irrigated cropland soils of dry regions
may increase rates of accumulation (Gislason et al., 1996; Nordt et
al., 2000). In contrast, secondary carbonates formed in soils having calcareous
parent materials (e.g., limestone) are neither a sink nor a source of CO2.

In humid regions, losses of SIC from naturally calcareous (e.g., limestone-derived)
soils can be accelerated through biomass removal and the use of acidifying
fertilizers in cropland and grassland (van Bremen and Protz, 1988). The use
of carbonate-rich groundwater for irrigation can also result in a net flux
of CO2 from groundwater storage to the atmosphere (Schlesinger, 1999).